DK Guard Ring Polarographic Cell

Transcription

1 REHDER DEVELOPMENT COMPANY 2139 Klondike Rd W Lafayette, IN USA Tel: (765) Fax: (765) Web: t.houck@rehder-dev.com Technical Innovations DK Guard Ring Polarographic Cell Instruction Manual Sept 19, 2013 Permeometer is a trademark of Createch. 1 of 8

2 REHDER POLAROGRAPHIC CELLS Rehder Polarographic Cells are designed to function using standard polarographic measuring procedures as described in the March 1982, March 1984 and October 1987 issues of the International Contact Lens Clinic. Great care must be taken to ensure that the 24 karat gold and pure silver electrodes do not become scratched. The Rehder Guard Ring Polarographic Cell enables polarographic readings to be taken without the error created by edge effect. This eliminates the need to calculate the value of the edge effect and to adjust your data. These cells are designed to function using standard polarographic measuring procedures. During normal use the surface of the silver will become coated with Silver Chloride. Cleaning after every 8 or 10 lenses should be sufficient to keep the cell in proper working condition. The tarnished surface may be removed by gently rubbing with a soft, high quality, facial tissue and fine silver cleaning polish. Caution: After polishing, you must thoroughly remove any "Tarnish Guard", or any substance designed to remain on the surface to prevent tarnishing, from the Cell working surface before it is put into service. Ethanol may be used on the Cell working surface but should be avoided on the Cell base block. After polishing, the cell should be thoroughly rinsed with water before it is used to take polarographic measurements. Make sure that the polarographic amplifier is turned off or that the Cell is disconnected from the amplifier prior to polishing the Cell. This will help to avoid plating the surface of the gold electrode with silver. Excessive polishing will wear away the working surface of the cell and may expose an increased amount of the gold surface. If the cell becomes excessively worn it should be resurfaced or replaced. All Guard Ring Cells are fitted with a solid state temperature sensor embedded within the silver anode of each Cell. The sensors are temperature calibrated to operate with the temperature readout on the Createch 201T Permeometer. Each Cell has a connector attached to mate with the Permeometer and is ready for use. The new cells have a larger base block and will not fit into the old cell mounting fixtures. Older cells have a smaller base block and will require an adapter to fit properly in the new mounting fixtures. Radius Cell Flat Cell Permeometer is a trademark of Createch. 2 of 8

3 The switch found on the calibration box, about mid way along the lead from the cell, is used to select different options for the current produced by the guard ring. If the guard ring switch is ON the meter reading shows the sum of the currents produced by the cathode and the guard ring. (As if the cell has a 5.5mm diameter cathode.) If the switch is in the OFF position, the guard ring is inactive and no current flows through that element. The cell then performs like a standard cell without a guard ring. If the switch is in the GND (Ground) position, the guard ring is active, current flows through the element, but the current is not read by the meter. This is the condition in which the guard ring eliminates the edge effect from the cathode. Permeometer is a trademark of Createch. 3 of 8

4 DETERMINING THE "DARK CURRENT" OF REHDER POLAROGRAPHIC CELLS The "Dark Current" value should be subtracted from current readings taken during Transmissibility measurements. The "Offset" dial on the Permeometer may be used to automatically subtract this value. 1. Set up as illustrated below and put the Cell in the mounting fixture, compressing the "sandwich" with the plunger. 2. Put the Cell and Fixture in your 35 C 3. Plug Cell into the Permeometer. 4. Switch on the Permeometer and make sure the "Offset" dial is set to ZERO. 5. Run the Cell until the oxygen in the cigarette paper has been consumed and the current reading stabilizes. (This may take 45 minutes or longer) A chart recorder plugged into the Permeometer will make it easier to monitor this process. 6. The stabilized current reading is the "Dark Current" for the Cell and each Cell's value should remain constant. Record this value for future use. You should expect a "Dark Current" reading of approximately 0.10 µa to 0.20 µa. Note: Only nonpreserved saline solution should be used on your samples or on the polarographic cell. (Unisol or equivalent is satisfactory). Preserved saline solutions may contain mercury compounds that will interfere with the polarographic electrochemical reduction of oxygen at the cathode. The mercury compound may also attack the silver anode. Aluminum Foil PMMA Lens Nonpreserved Saline Saturated Cigarette Paper Nonpreserved Saline Saturated Cigarette Paper PMMA Lens Flat Cell Glass Microscope Slide Radius Cell Permeometer is a trademark of Createch. 4 of 8

5 REHDER DEVELOPMENT COMPANY 2139 Klondike Rd W Lafayette, IN USA Tel: (765) Fax: (765) Web: rehder - dev. com t.houck@ rehder - dev. Technical Innovations com For Flat Cell Strech nylon mesh (panty hose type) over tube ends and secure with thread in groove. Nylon mesh on tube end holds sample onto Cell For Radius Cell Sample Mounting Position Radius Cell Cell Mounting Fixture Flat Cell Radius Cell Permeometer is a trademark of Createch. 5 of 8

6 Cleaning the Polarographic Cells: During normal operation of the polarographic cells the surface of the silver anode will become coated with silver chloride. The silver chloride coating will appear as a dull gray, tarnished, surface on the anode. If the silver chloride coating is allowed to build up on the anode the cell will not perform properly. Therefore, you must polish off the silver chloride coating to expose the clean surface of the silver. It is important that the cell be disconnected from the Permeometer or that the Permeometer is turned off during the polishing operation. If the current is turned on to the cell while polishing, a coating of silver will be Plated onto the gold cathode. You should continue polishing until the tarnished surface has been completely removed from the anode and any silver deposits on the gold have been removed. When finished, you should see bright silver and bright gold. I use two types of polish when cleaning tarnished cells. The first, is the Boston Lens cleaning polish which is only very, mildly abrasive. The second, which is slightly more abrasive, is a metal polishing product called Simichrome Polish. This polish is manufactured in Germany and can be found in automotive and hardware stores. I think that most fine silver polishes should work OK. You should test the polish on the outside of the cell to make sure there is not an adverse chemical reaction with the insulating material before using the polish on the cell surface. You can also use the silver polishing wadding material. Caution: Do Not use polish that includes any type of "Tarnish Guard" or any substance designed to remain on the surface to prevent tarnishing. Please keep in mind that these cells are consumable items. Each time the silver chloride is removed, a small amount of the surface of the silver disappears. Eventually, the silver surface will be eroded to the point where the cell surface should be remachined. Depending on the eroded condition of the cell, and some other factors, a cell may not be restorable. The silver anode on the polarographic cell is several millimeters thick. However, the temperature sensor is located within the silver anode and is very near the surface of the cell in order to provide quick response and good resolution. The position of the temperature sensor and the thickness of the gold usually allows a cell surface to be remachined one time before it is no longer usable, this, of course, depends on the amount of erosion. If the cell becomes excessively eroded it should be discarded. It should take hundreds of lenses being tested to reach the point where the cell needs to be remachined. I would recommend remachining if the silver on the cell surface becomes eroded by about 150 microns. The gold cathode will not be eroded nearly as quickly as the silver. If you notice that the gold is protruding slightly above the radius of the silver, then it is probably time to have the cell remachined. Permeometer is a trademark of Createch. 6 of 8

7 Set Up Instructions for Dk/L Measurements: Three papers from the International Contact Lens Clinic, March 1982, March 1984 and October 1987, that were authored by Dr. Irving Fatt and others, should help you understand the process of determining Dk/L. Setting up the equipment is not complicated. The operating instructions for the Electronic Thickness Gauge and Permeometer should help you get things set up. You can also refer to information in the web site and information found in the ISO standard. The Guard Ring Cell plugs into the front of the Permeometer in either the "A" or "B" position. The cell has the temperature sensor built in and temperature of the Cell will be displayed on the Permeometer. You must select the A or B channel on the Permeometer. If you have purchased the computer interface and software so you will need to follow the instructions for connecting the computer and installing the software. By using the computer, you will not need to use a chart recorder to track oxygen consumption or track temperature so you will not need the two phone plugs that were supplied. The phone plug connectors will plug into the rear of the Permeometer if you do choose to use a chart recorder. You will need to attach some nylon mesh to the end of the tube on the Cell Mounting Fixture. The mesh holds the sample in place on the Cell. Refer to the diagram in the Cell Operating Instructions that shows how the mesh is installed and how the sample is mounted. The Mounting Fixture with the sample mounted, needs to be placed in an environmental chamber to control temperature and humidity. Prior to mounting, the Sample, the Fixture and Cell should all be stored in the chamber at the test temperature. This will help to avoid long delays in bringing the sample to the desired temperature for testing. Dr. Fatt usually kept all the vials with his samples to be tested in the chamber so everything was at temperature and ready for testing. The Electronic Thickness Gauge is used to measure the thickness of the sample. It is a stand alone instrument, it has been calibrated, and is ready for use. You can determine "Transmissibility" of your sample from the data taken from the Permeometer. To determine "Permeability" of the material you must know the thickness. Permeometer is a trademark of Createch. 7 of 8

8 Set Up for Dk Measurements with Computer Interface Incubator Cell with Sample mounted in Fixture Inside Incubator Permeometer With Computer Interface Computer Permeometer is a trademark of Createch. 8 of 8